LED lamps and luminaries are more and more penetrating the classical market segment of fluorescent and TL-Lamp applications.
Known LED driver circuits include two or three individual power converters to operate LEDs with a DC current from AC mains grid. Thus, they have a high component count and moderate efficiency of about 80%. Applications with a power level above 25 W usually include a second DC/DC boost converter to fulfill AC mains current harmonic standards, thus further increasing component count.
A low component count and high efficiency is also of interest for the inverse transfer of power, for instance the transfer of a DC current from solar cells into an AC grid. A dual buck-boost DC/AC inverter is known from N. Vazquez et al., “Analysis and experimental study of the buck, boost and buck-boost inverters”, Proceedings of the IEEE Power Electronics Specialist Conference, 1999, pp. 801-806. However, this circuit has disadvantages due to its control principle. In particular, the two modulated DC voltages have a high peak value and there is a high internal power flow leading to a low efficiency.
The principle operation of a single buck-boost DC/DC converter is known from Mohan, Underland, Robins, “Power Electronics: Converters, Applications and Design”, John Wiley & Sons, ISBN 0-471-50537-4, 1989. A 60 W buck-boost DC/DC converter with an efficiency of up to 98% is disclosed in W. Zhou, T. Philips, “Industry's First 4-Switch Buck-Boost Controller Achieves Highest Efficiency Using a Single Inductor”, Linear Technology Corporation, Design Note 369, 2005.
US 2007/0058402 A1 discloses a synchronous full bridge rectifier which is controlled to provide a power factor near unity. The full bridge rectifiers are transistors, each with a controlling input. The AC input signal and currents within the circuit are sensed and sent to a controller. In response, the controller outputs control signals to turn on/off the rectifying MOSFETs timely to form a power factor of near one with respect to the AC input signal. The full wave rectifier is made of N-channel MOSFETs, some with fast body diodes. The MOSFETs are rectifiers and power factor circuit (PFC) control elements. The result is a one-stage synchronous rectifier with PFC. A solid state precision analogue differential amplifier senses the AC line waveform and high frequency current transformers sense the currents. The controller accepts the inputs of the amplifier and the sensed currents and outputs control signals that turn on and off the four MOSFETs. The timing of turning on/off is arranged so that the current drawn from the AC source is sinusoidal and matches the phase of the sinusoidal AC source. This special boost converter circuit generates a DC output voltage that is higher than the input voltage. For supplying a number of LEDs or OLEDs coupled in series, this circuit is not suited, however, since LEDs and OLEDs have a limited DC supply bus voltage. High DC bus voltages generate high electric fields in some series-connected LEDs or OLEDs that can damage these devices.